Conventionally, cold cathode tube lamps are used as a light source in various devices. For their low power consumption and long life spans as a light source, cold cathode tube lamps are used for a light source (backlight) in, for example, liquid crystal display devices and the like (see, for example, Patent Document 1 listed below).
FIG. 14 is a sectional view showing structure of a conventional cold cathode tube lamp. FIG. 15 is a sectional view taken along line 500-500 in FIG. 14. Referring to FIGS. 14 and 15, the conventional cold cathode tube lamp will now be described.
As shown in FIG. 14, a conventional cold cathode tube lamp is provided with a glass tube 501, and with electrodes 502 and 503 forming a pair of cold cathodes arranged opposite each other inside the glass tube 501 at opposite ends thereof. Lead terminals 504 and 505 are connected to the electrodes 502 and 503 respectively. As shown in FIG. 14, the lead terminal 504 is, at one end, connected to the electrode 502 and is, at the other end, brought out of the glass tube 501. Likewise, the lead terminal 505 is, at one end, connected to the electrode 503 and is, at the other end, brought out of the glass tube 501. The glass tube 501 is airtightly sealed and hermetically closed by the lead terminals 504 and 505. Although unillustrated, a fluorescent substance is applied to the interior wall of the glass tube 501, and rare gas, such as argon or neon, along with mercury is, as discharge gas, sealed inside the glass tube 501.
In a cold cathode tube lamp as described above, when predetermined voltage is applied between the electrodes 502 and 503 via the lead terminals 504 and 505, a tiny number of electrons present inside the glass tube 501 are attracted to and collide with the electrode. As a result, from the electrode hit by electrons, secondary electrons are emitted, starting electric discharge. The emitted electrons collide with the mercury inside the glass tube 501, and the collision of electrons with mercury produces ultraviolet radiation. The ultraviolet radiation excites the fluorescent substance applied to the interior surface of the glass tube 501, causing visible rays to be emitted.
After use for a long time, the cold cathode tube lamp as described above suffers a phenomenon (sputtering) in which ions or the like colliding with the electrode 502 (503) expel atoms from metal material forming the electrode 502 (503). The atoms (sputtered matter) of the electrode metal expelled by the sputtering combine with the mercury inside the glass tube 501, and thus the mercury to be used for electric discharge is consumed, leading to a problem of diminished luminance in the cold cathode tube lamp. Diminished luminance in the cold cathode tube lamp makes it unusable as a backlight, counting as a factor in shorter life spans. Moreover, in the conventional cold cathode tube lamp as shown in FIG. 14, collision of ions or the like concentrates on the interior surface of a bottom part of the electrode 502 (503), and this produces a hole penetrating the bottom part of the electrode 502 (503), or in some cases even make the electrode 502 (503) drop off, thus resulting in breakage of the electrode 502 (503).
In the meantime, in recent years, further improvements have been sought in the backlights for lower power consumption, longer life spans, higher efficiency, etc. For example, as a way to solve the above-mentioned problem of shorter life spans resulting from the sputtering, a method is known which involves increasing the gas pressure inside the glass tube 501 of cold cathode tube lamp. Increasing the gas pressure inside the glass tube 501, by reducing the movement speed of electrons and otherwise, helps suppress the sputtering, and thus helps suppress shortening of the life span. While helpful in sustaining the life span of cold cathode tube lamp, however, increasing the gas pressure inside the glass tube 501 has disadvantage of lower light emission efficiency, and hence lower luminance. Moreover, increasing the gas pressure inside the glass tube 501 increases an energy loss resulting from the collision of electrons, and thus has disadvantage of increased heat generation as well. The increased heat generation raises tube wall temperature of the glass tube 501, and this causes a phenomenon of released ultraviolet radiation being re-absorbed by mercury, leading to the problem of lower light emission efficiency.
On the other hand, it is known that reducing the gas pressure inside the glass tube 501 and passing large electric current helps improve the light emission efficiency. Reducing the gas pressure inside the glass tube 501, however, by increasing the movement speed of electrons, makes the sputtering more likely to occur, leading to the problem of shorter life spans. One possible way to solve this problem is to increase tube diameter of the glass tube 501. Increasing the tube diameter of the glass tube 501 reduces electric current density over a discharge area on the electrode 502 (503) and makes the sputtering less likely to occur; doing so, thus helps suppress breakage of the electrode 502 (503) resulting from the sputtering, and thereby helps suppress shortening of the life span.    Patent Document 1: JP-A-2002-289139